I think you can find the answer here and here.

In short, LegacyOS (or Euclid) is used to built geometry for E+ not for Open Studio models. And yes, there's a plug-in called OpenStudio Plug-In for Sketchup. It comes with the OpenStudio download. Here are the official is intro tutorialstutorial. If you don't see it under the Extension dropdown menu, check Check Preferences > Extensions on whether it is enabled and disable the LegacyOS Extension.Extension.